xHCI/USB: Make xHCI driver have a BOS descriptor.

To add USB 3.0 link power management (LPM), we need to know what the U1
and U2 exit latencies are for the xHCI host controller.  External USB 3.0
hubs report these values through the SuperSpeed Capabilities descriptor in
the BOS descriptor.  Make the USB 3.0 roothub for the xHCI host behave
like an external hub and return the BOS descriptors.

The U1 and U2 exit latencies will vary across each host controller, so we
need to dynamically fill those values in by reading the exit latencies out
of the xHC registers.  Make the roothub code in the USB core handle
hub_control() returning the length of the data copied.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>